The invention relates to a method for measuring the vapor pressure of liquids in which method the liquid to be measured is placed into a previously evacuated measuring cell and the effect of the gases dissolved in the liquid on the measurement results is excluded. The invention, further, relates to an arrangement for measuring the vapor pressure of liquids.
The vapor pressure of a liquid is that pressure which in a closed system builds up above the liquid; this pressure changes until the vapor is in dynamic equilibrium with the liquid. The then-present pressure is called saturation vapor pressure and this is exclusively a function of the temperature of the system and does not depend on its geometry. The values published in the literature for the vapor pressure of a substance are always saturation vapor pressure values, since only these can be stated specifically for the substance and do not depend on the measuring method. In principle, the measurement of the vapor pressure of a liquid can be carried out so that this liquid is placed into a tightly closable vessel which is provided with a pressure measuring arrangement, the vessel closed, and the pressure present in the vessel is measured. If the quantity of liquid is sufficiently large so that at the measuring temperature in spite of evaporation residual liquid remains in the vessel, the vapor pressure is the saturation vapor pressure. These circumstances, which in principle are simple, are complicated by the fact that the ambient atmospheric pressure and the pressure of the gas dissolved in the liquid to be measured can significantly influence the measurement.
The techniques used until now either accept a falsification of the measured results through the ambient atmospheric pressure and the pressure of the gas dissolved in the liquid, with in some cases the aim being to keep the error approximately the same through standardized procedural specifications in order to be able to compare the measuring results obtained from different liquids (for example vapor pressure determination after Reid), or the aim is with extremely great operation complexity in terms of laboratory means to exclude not only the influence of the ambient atmospheric pressure but also to free the liquid to be tested of the gases dissolved therein before undertaking measurements. With the known techniques particularly great difficulties are encountered if the vapor pressure of mixtures of liquids is to be determined.
In GB-A-832 085 a method for measuring the vapor pressure of liquids is described in which first the liquid to be tested is placed into an evacuated chamber in which grainy material is located in order to make available a large surface. The large surface favors the emergence of gases dissolved in the liquid. Following this pretreatment, the liquid is transferred to a bottle, the evacuated chamber is cleaned, and the liquid is subsequently placed once again into said chamber which thereupon is again evacuated and subsequently closed, whereupon the pressure developing in this chamber is measured. This approach is cumbersome, and in the case of liquid mixtures changes in the composition result through the evacuation. It is in GB-A-832 085 further described that the quantity of a gas contained in a liquid can be determined if the vapor pressure of this liquid can be neglected, in that this liquid is placed into an evacuated chamber in which grainy material is present, and measures the pressure built up in this chamber by the gas emerging from the liquid. From the thereby, obtained measured values for the pressure using the relation p*v=R*T, the quantity of the gas which had been present in the liquid relative to normal pressure and normal temperature is calculated.
A method for determining the gas content of a liquid is also described in GB-A-974 183. In this method too, the liquid to be tested is placed into an evacuated chamber, and the pressure originating in this chamber is measured. The volume of the chamber in which the liquid is located is variable by displacing a piston. Two measurements at different positions of the piston are carried out. The measured pressure is composed in each instance from the pressure of the gas and the vapor pressure of the liquid. From two measured pressure values, assuming that the unknown vapor pressure has the same value in both measurements, the gas quantity contained in the liquid is calculated.